Passive supports such as footers, piles, and caissons are well known subsurface supports for many man-made structures such as bridges, buildings, and the like. These supports may be characterized as “passive” because the earth surrounding the supports must first shift or move to mobilize the available tensile, bending, and/or shear capacities of the supports.
In addition to passive subsurface supports, more recently, it is known to provide ground strengthening by driving elongate reinforcing members, referred to as soil nails, into the ground under and/or adjacent to structures in order to improve the bulk properties of the soil/rock formation that supports the overhead structure. Typically, soil nails are provided in a predetermined array to target improvement of the soil/rock formation at specified locations. Soil nails themselves are not used for direct support of the overhead structure; rather, the soil nails are used to prevent shifting or other undesirable properties or characteristics of the particular geological formation upon which the structure is built.
For methods of supporting ground excavations, excavations supports or shoring can be broadly classified as external and internal. External support methods relate to support provided outside the confines of the excavation. Examples of external supports include berms, rakers, cross-lot bracing, anchors, and cantilever walls. Internal support methods are those methods that provide support by reinforcement directly into the existing ground. Examples of internal supports include the use of soil nails and micropiles.
Soil nail installations may also be generally categorized within two general types. A first type includes soil nail installations that use a solid bar soil nail according to a “drill and grout” method. This method is most efficient in soils where open-hole drilling is possible. However, within caving ground conditions, such as loose soils with cobbles and raveling or running sands, a casing may be required to support the drilled hole. Use of casing substantially slows a soil nailing process, and clearly adds to cost. Therefore, in most circumstances, casings are avoided. The other general type of soil nail installation involves the use of a hollow core soil nail in which an oversized sacrificial drill bit is used as a cutting tool to advance the hole. The drill bit includes a plurality of holes or passageways that communicate with the hollow core of the attached soil nail. The soil nail is rotated along with the drill bit during installation, and is advanced using force applied by, for example, a percussion hammer. Once the hollow core soil nail bar is advanced to a desired depth in the drilled hole, it is left in the hole along with the drill bit. Grout is then pumped at high pressure through the hollow core of the soil nail and through the drill bit. Ultimately, grout pressure forces the grout back along the outside surfaces of the soil nail bar and towards the surface to fill the drilled hole. The hollow core soil nail bar therefore acts as a grouting conduit in addition to its primary purpose as a subsurface reinforcement element. In many cases, the simultaneous actions of drilling the hole, installing the soil nail, and grouting the nail within the hole is more efficient than the conventional “drill and the grout” method of installation, and is certainly more efficient than the conventional method of installation requiring use of a casing within the drilled hole.
Another specific advantage of a soil nail installation using hollow core soil nails and a sacrificial drill bit with grout conveying passageways, is that a better “grout to ground” bond may be achieved. The dynamic rotary pressure grouting characteristic of the method enables the grout to better permeate the geo-material surrounding the drilled hole as compared to the “drill and grout” method. Improved permeation into the surrounding geo-material results in an improved bond between the grout and the geo-materials. The area into which penetration of the grout occurs into the geo-material is referred to as a permeation zone. The permeation zone may vary between soil types, but nonetheless, the pressurized grouting aspect of the hollow core soil nail method appears to improve the thickness of the permeation zone for all soil types. An increased permeation zone directly improves the pullout resistance or capacity of the soil nail installed. Additionally, this method also provides improved stiffness load deformation capacities that can be observed during pullout testing of an installed hollow core soil nail.
Various accessories are used with soil nail installations. One often used accessory is a bearing plate that is mounted to the exposed end of the soil nail. The bearing plate provides a compression force against the exposed surface of the excavation, and serves to stabilize the soil nail in its installed orientation. Particularly for relatively loose and caving soils, a bearing plate is selected in a size to ensure an adequate amount of pressure can be distributed across an area of the exposed surface of the soil to keep the soil nail in place without appreciable shifting.
Another accessory commonly used is a “centralizer”, and this accessory is used to centralize the soil nail in the drilled hole so that an even distribution of grout can be achieved circumferentially around the soil nail. A misaligned or off-center soil nail results in at least one side of the soil nail being placed in close proximity to the surrounding geomaterial, thereby resulting in a poor grout to ground bond at that location. The soil nail is likely to prematurely rust or corrode due to its closer proximity to moisture in the geomaterial. For hollow core soil nails that are rotated along with the sacrificial drill bit during installation, the current solution is to provide a “mobile centralizer” that is loosely mounted over a desired section of the soil nail. The intended operation for these centralizers is to allow them to freely rotate and move along the length of the soil nail during the installation process. A typical example of a mobile centralizer is one that has an inside diameter greater than the outside diameter of the hollow core bar, but a smaller outside diameter as compared to the outside diameter of a coupler used to interconnect adjacent sections of a soil nail. A common shape for these centralizers is a ring shaped body and a plurality of spacers that extend radially outward from the body. The spacers provide the centering capability for keeping the soil nail centered within the hole.
While these mobile centralizers may be adequate for their intended purposes in many installations, there are also some limitations associated with use of such mobile centralizers. Due to the oversized interior diameter of the ring shaped body, the centralizer itself can become jammed and held against a surface of the bar over which it is mounted. This jammed orientation is caused by a partial rotation of the centralizer with respect to the longitudinal axis of the soil nail such that the spacers of the centralizer are not oriented perpendicular to the soil nail and therefore, do not keep the soil nail centered within the drilled hole. Additionally, mobile centralizers are limited in size—their diameter cannot exceed the diameter of the drill bit because a mobile centralizer with a diameter greater than the drill bit will inevitably become jammed in the hole, thus preventing advancement of the drill bit, and possibly resulting in damage to the soil nail assembly as it continues to rotate. Mobile centralizers are subject to whatever forces are present within the drilled hole, and the centralizers cannot be precisely positioned along any certain point over the soil nail. Without consistent spacing between centralizers, a soil nail may not be optimally centered in the drilled hole. A disadvantage associated with commercially available centralizers is that they are not made of steel like the couplers and soil nails. Because of the relatively complex shape of the mobile centralizers, and perhaps for cost reasons, they are cast. For example, many mobile centralizers are made from a cast iron coated material known to corrode more quickly than the soil nail sections and coupler. Use of a cast iron centralizer with steel soil nail sections and couplers also results in a dissimilar metal environment within the drilled hole. The dissimilar metals can cause a galvanic reaction that accelerates corrosion of the coupler and soil nail sections.
In excavations for many projects, there can be distinct layers of geo-material encountered. For example, in landslide areas, the upper soil layer may comprise relatively loose fine sands, and small rocks that have a low bond strength with an installed soil nail. In this example, the length of the nail must be extended such that the distal end or lower portion of the soil nail penetrates into denser geo-material under the landslide debris. The extension is typically achieved with a coupler that interconnects two sections of soil nails. The specified bond strength for the installation may be primarily dependent upon on the lower portion of the soil nail penetrating the denser geo-material. The proximal or upper portion of the soil nail may still require a larger than normal bearing plate in order to compensate for the reduced bond strength by increasing bearing capacity applied by the plate to the upper layer of loose soil. It is clear that the overall cost and complexity of an installed soil nail increases in this case because the bearing plate must be oversized.
It should therefore be apparent that there are many unmet needs associated with soil nail assemblies, soil nail accessories, and methods of emplacement.